The removal of particulates from a gas stream has long been a practice in a variety of industrial fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes, filter panels and filter cartridges. For convenience herein, the term “filter element” will be used to refer collectively to these types of filtration means.
Selection of the type of filtration media used is typically based on the fluid stream with which the filter element comes in contact, the operating conditions of the system and the type of particulates being filtered.
The flow of fluids, whether liquid or gas, produces a pressure differential, or pressure drop, across the element. Preferably, the pressure differential is as small as possible for a given fluid flow rate in order to minimize the power required to filter the fluid. Over time, however, the pressure drop can increase as filtered particles accumulate on the filter element. When the limits of acceptable pressure differential or flow rate reduction have been reached, the filter element must be either replaced or cleaned.
Filter media may be broadly characterized as either depth filtration media or surface filtration media. Particles tend to penetrate somewhat and accumulate within depth filtration media. In contrast, the majority of particles collect on the surface of surface filtration media.
Many materials are known to be useful as depth filtration media, including spun bond or melt blown webs, felts and fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, cellulose, glasses and fluoropolymers. Known melt blown filter media demonstrate high efficiency and low pressure drop. Melt blown filter media also have high dust capacity. However, melt blown filter media suffer from relatively low water entry pressures, which may make them unsuitable for outdoor use in some environments.
Providing a static electric charge to depth filtration media such as melt blown media improves its filtration efficiency. Electrostatic filter materials, or electrets, have electrostatically enhanced fibers which enhance filter performance by attracting particles to the fibers, and retaining them. Electrostatic filters rely on charged particles to dramatically increase collection efficiency for a given pressure drop across a filter. Pressure drop in an electrostatic filter also generally increases at a slower rate than it does in a mechanical filter of similar efficiency.
Electrostatic media may lose efficiency during use, particularly when used in an environment in which the filter element is exposed to moisture or oily particles. Many of the particles and contaminants with which electrostatic filters come into contact interfere with their filtering capabilities. Liquid aerosols, for example, particularly oily aerosols, tend to cause electret filters to lose their electrostatically-enhanced filtering efficiency.
To reduce these effects, the amount of the non-woven polymeric web in the electret filter may be increased by adding layers of web or increasing the thickness of the electret filter web. The additional web, however, increases the pressure drop across the electret filter and adds weight and bulk.
Surface filters, such as membranes, have gained popularity in certain applications, particularly outdoor environments or those in which the fluid to be filtered contains liquid aerosols or harsh chemicals. In other applications, membrane filter media is useful because it has a more constant filtration efficiency than that of depth filtration media. Membranes have stable filtration efficiency because, unlike depth filtration media, a membrane filter's efficiency is not dependent upon the buildup of a cake of dust particles.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas such as harsh chemical environments, which normally degrade many conventional metals and polymeric materials. A significant development in the area of particle filtration was achieved when expanded PTFE (ePTFE) membrane filtration media were incorporated as surface laminates on conventional filter elements. Examples of such filtration media are taught in U.S. Pat. No. 4,878,930, and U.S. Pat. No. 5,207,812, which are directed to filter cartridges for removing particles of dust from a stream of moving gas or air. Membranes constructed of ePTFE are advantageously water tight. However, membranes may exhibit relatively high pressure drop when compared to depth filtration media and have relatively low dust capacity. Accordingly, in some applications, filter elements using membranes will need frequent replacement or cleaning.